1. Field of the Invention
The present invention relates to a monitoring and current measuring unit for MOSFET transistors in low side (LS) and/or high side (HS) configuration, with any electrical load, in particular, an inductive load. The invention is aimed at a precise detection of a current through a transistor and at the detection of faulty conditions by only observing the voltage curve of the drain-to-source voltage of a HS and a LS transistor for driving a load.
2. Description of Related Art
Conventionally, to realize the functionality described, a measuring method is frequently used for load drives with discrete power MOSFETs, such a method is based on measuring current across a shunt resistor in the load current circuit (see, e.g., U.S. Pat. No. 4,967,309 and U.S. Pat. No. 4,820, 968). At present, these discrete power MOSFETs are driven by ICs based on high voltage BCD and CMOS process technologies. With high load currents, it is not economic to integrate the shunt resistor into the drive element. Therefore, power MOSFETs are used that have a sense current output through, which a part of the load current is coupled. Then, the shunt resistor may be integrated into the drive element, thus saving external components. This advantage comes at the price of needing a greater functional effort for the discrete power MOSFETs. With fully integrated LS/HS switch configurations based on junction-insulated BCD or CMOS process technologies, this greater effort is negligible. However, a drawback to the above is that the power MOSFETs are produced in a rather complex process technology when compared with discrete transistors so that this solution is no longer economical above a certain current value. Thus, current measuring, e.g. in a motor bridge circuit comprising discrete power MOSFETs without shunt resistors, based on the voltage drop across the drain-to-source path, is desirable. Since the turn-on resistance of a transistor is subject to a relatively large variance in components, temperature dependence, and degradation with age, this measure is very inexact so that it is insufficient for a control and monitoring of a motor bridge circuit, for example. Calibration methods based on calibration curves of the turn-on resistance as a function of the temperature are elaborate and costly.
In German Patent 197 43 346, a circuit arrangement for clocked current control of inductive loads is known, wherein a free-wheeling diode with a current measuring means is connected in parallel with the load. In this circuit arrangement, no shunt resistor is needed since current measuring is effected exclusively during the OFF state of the switch controlling the load.
Finally, in German Patent 197 36 752, another circuit arrangement is known for protecting a drive circuit, wherein the voltage drop across a driving field effect transistor is measured and evaluated by a microprocessor. In this circuit arrangement, the determination of the load current requires knowledge of the internal resistance of the field effect transistor. However, this internal resistance is not constant over the operating area of the field effect transistor, which is why measuring with the known circuit arrangement is rather inaccurate and established protection of the driving circuit is very limited.
In WO-A-98/10301, the current in the transistor is measured by observing the voltage drop induced by the load current across the transistor. The temperature of the transistor is measured by observing the junction voltage of a diode that is biased with a test current source and thermally connected to the transistor. This temperature indicating diode voltage is translated into a predicted transistor on-resistance. By dividing the measured voltage drop across the transistor by the predicted on-resistance, the magnitude of current provided to the load is determined. The current measurement requires a calibration measurement of the on-resistance at a known temperature. This leads to additional effort and if it is done only once in the production line device, degradation will not be taken into account. Having a diode that must be thermally coupled to the transistor increases the production cost.
In DE-A-197 04 861 A1, circuitry and method for controlling and observing semiconductors switches is described. Like in WO-A-98/10301, the drain source voltage drop induced by the load current is observed. The temperature is measured with bipolar transistor thermally connected to the MOSFET carrying the load current. The maximum drain-source voltage is determined by taking into account the thermal resistance between junction and the temperature at the measurement point. Like in WO-A-98/10301, the temperature measuring device and the calibration of the MOSFET on-resistance increases the production cost. In the present invention, the temperature is measured directly at the junction carrying the load current. This is more safer, more precise and less expensive as compared to measuring the diode forward voltage of a device which is only indirectly thermally coupled, like in the PCT mentioned above.
In U.S. Pat. No. 4,945,445, the current in the semiconductor switch is measured as a voltage drop across the bond wire with two additional current sense pads connected by a conductor jumper bond wire. The current sense circuitry observes the voltage drop across the current sense pads and amplifies this voltage.
A comparator compares the amplified voltage against a reference voltage and generates an output signal when the threshold is exceeded. A second comparator compares the thermistor voltage against a reference voltage and generates an output signal when the temperature exceeds the threshold. Both output signals are used to disable the gate drive of the MOSFET. This solution needs a special five terminal MOSFET module housing and a thermistor element for the temperature measurement. Due to the non-standard module housing and the thermistor element this solution is more expensive than the solution in the present invention. The calibration of the bond wire resistance requires additional effort.
In U.S. Pat. No. 5,804,979, a circuit for measuring alternating and direct current without breaking the conductor is provided. A modulated current source is used to couple a test current across the segment of the conductor. A synchronous demodulator which is also coupled in parallel with the segment separates the voltage drop due to the modulated test current from the current in the conductor. By measuring the test voltage drop and the voltage drop and combining this with the test current, the resistance of the segment and the current in the segment can be calculated. The described measurement principle is applied to an linear resistance connected to a current source. The requirements for monitoring and controlling a semiconductor switch and its different load conditions are not dealt with. Furthermore thermal effects are not considered.
In a circuit arrangement for monitoring an electronic switch provided for controlling a load and load current flown through the load during turn-on intervals for driving the load, the switch has a substantially linear resistance behavior during parts of its turn-on state and the operating point of the switch is located in this part of the turn-on state during the load control. The circuit arrangement comprises a modulation signal generator for generating a modulation signal for modulating the load current flowing through the electronic switch, a first voltage measuring device for measuring the first voltage drop across the electronic switch caused by the modulation signal, a second voltage measuring device for measuring the voltage drop across the electronic switch caused by the load current, and an evaluation unit for determining the load current from the modulation signal and the first and second voltage drops.
The invention to be described could find application, for example, in a coil driver of an electromagnetic valve. In this case, the coil driver may be designed as a LS switch and/or a HS switch. Other possible applications are a MOSFET bridge circuit for driving pulsed DC motors, stepper motors, or asynchronous motors. A precise detection of the current-voltage curve is necessary for current control and for fault diagnosis. Using, for example, an integrated circuit (ASIC) for driving a half-/full-bridge circuit formed of external power semiconductors of the field effect transistor type, the above described arrangement can monitor occurring load current peaks and the internal heating-up of the power switches by exactly determining the RDS on-resistance and the switch voltage.
It is an objective of the present invention to provide a circuit arrangement for monitoring and controlling, i.e. supervising, an electronic switch that controls a load and through which a load current flows for driving the load, wherein the circuit arrangement can easily be connected to the terminals of a standard switch, a 3 terminal MOSFET, for example. No modifications of said electronic switch are required and the switch is operable without using a shunt resistor or any additional temperature measuring devices.
According to the present invention, the objective is solved with a circuit arrangement for monitoring an electronic switch provided for controlling a load and a current flown through the load during turn-on intervals and for driving the load. The switch has a substantially linear resistance behavior during parts of its turn-on state and the operating point of the switch is located in this part of the turn-on state during the load control. The circuit arrangement having a modulation signal generator for generating a modulation signal modulating the load current flowing through the electronic switch, a first voltage measuring device for measuring the first voltage drop across the electronic switch caused by the modulation signal, a second voltage measuring device for measuring the voltage drop across the electronic switch caused by the load current, and an evaluation unit determining the load current from the modulation signal and the first and second voltage drops.
The present invention starts from a low side/high side (LS/HS) switch configuration consisting of power MOSFETs with an inductive load, for example, and is directed to the realization of a circuit for load current measuring based on the drain-to-source voltage of the MOSFET switches. Of importance for the invention is the reduction of the current measured to a determination of the differential turn-on resistance of the active LS/HS transistors. The differential resistance of the turned-on transistor can be determined by measuring the change in the drain-to-source voltage caused by a known change of the current flowing through the switch (current modulation). When the transistor is in an approximately linear operating region, i.e. the difference between the gate-to-source voltage and the threshold voltage is larger than the drain-to-source voltage, the differential turn-on resistance and the absolute value of the turn-on resistance are identical. Thus, the unknown load current can be determined from the absolute value of the drain-to-source voltage and the differential turn-on resistance.
According to the invention, it is not imperative to determine the differential turn-on resistance separately. In determining the load current, the differential turn-on voltage merely serves as an intermediate value that does not necessarily have to be supplied by the evaluation unit as a value. The evaluation unit employs Ohm""s law to derive the (momentary) value of the load current from the modulation signal and the first and second voltage drops.
As far as supervisory purposes are concerned, the knowledge of the actual turn-on resistance of the electronic switch can be determined by the evaluation unit from the modulation signal and the first voltage drop, i.e. from the voltage drop across the turn-on resistance caused by the modulation of the load current.
Using prior calibration data of the electronic switch, the temperature variation of the switch can be determined as a function of the change in the turn-on resistance. By determining the turn-on resistance, the actual temperature of the electronic switch may be derived in the evaluation unit.
Generally, the electronic switch will be a field effect transistor, in particular a MOSFET.
The modulation signal, which, in particular, is a measuring current supplied into the connecting line between the electronic switch and the load and/or xe2x80x9cdrawnxe2x80x9d from this node, flows dependently on the ratio of the impedances of the load and the electronic switch across these two components. If the impedance (ON resistance) of the electronic switch in the operating point is negligible compared to the impedance of the load, the portion of the measuring current flowing off over the load may be neglected in the determination by the evaluation unit. For reasons of simplicity, the evaluation unit uses the measuring current generated by the measuring current source. This measuring current is known per se and can be obtained from the driving signal of the modulation signal generator, for example, which, in the present case, is a current source.
If, however, the impedance of the electronic switch is not negligible with respect to the impedance of the load, the portion of the measuring current flowing off over the electronic switch can be determined when the impedance of the load is known so that, in the evaluation unit, this portion of the measuring current passing the electronic switch is included in the calculation. As described above, the present device can be used to determine, during a turn-on interval, the electric current flowing through the closed electronic switch and also through the load itself. With the electronic switch turned off, the connection between the load and the operating voltage source is interrupted.
If, in this case, a modulation signal is generated, a current eventually flows through the load referenced above as the measuring current. With the switch opened, this measuring current flows through the load. Should the load impedance be unknown, but be substantially linear and, in particular, constant, the measuring current and the first voltage drop can be used to determine the momentaneous impedance of the load. If the supply voltage is known, it is included in the determination of the impedance. This supply or operating voltage may also be obtained during those intervals in which no current, due to the modulation signal, flows through the load, by measuring the second voltage drop. By this determination of the impedance of the load, a higher measuring accuracy of the present circuit arrangement can be achieved, specifically by making use of the impedance of the load, determined in the OFF state of the switch, in the subsequent determination of the load current with the switch closed.